This invention relates to electronic liquid detection circuits. Specifically, this invention relates to circuits for detecting the presence of a cryogenic liquid in a closed vessel. Fluid detection circuits generally sense the presence of a liquid by measuring either the electrical conductivity or the temperature of a liquid as an indicator of its presence. Sonic transducers have also been developed that measure the distance between a transducer and the surface of the fluid in a vessel, by measuring the time it takes a sound wave to travel to the fluid surface. In detecting the presence of a cryogenic liquid, commonly used with superconductors for example, the physical components used to sense the liquid must be able to operate at extremely low temperatures, near absolute zero. Since most liquified gases are non-conductive, a cryogenic liquid detector relying on electrical conductivity would be ineffective. Ultrasonic methods are not suitable because the holding vessels or cryostats used to store liquid gases, operate at widely varying pressures.
Existing fluid detectors for cryogenic liquids usually employ a method for sensing the temerature of the liquid as an indicator of its presence. A temperature sensitive resistance, such as a thermistor, that changes resistance proportionately with temperature, produces a detectable voltage change in response to a cold fluid surrounding or contacting it. Existing circuits using a temperature sensitive resistance such as a thermistor to detect a fluid, suffer on serious drawback: the inability to protect the sensing resistor from overheating when it's operated in a vacuum. Power must be removed from the sensing element before evacuating the vessel to protect the sensor from damage.
Cryostats, the vessels that hold cryogenic liquids, are frequency emptied and evacuated to purify the container prior to filling. In application where the liquid level in a vessel is automatically maintained, if a liquid detector used to automatically maintain the fluid level uses a temperature sensitive resistance that is carrying current is inadvertently destroyed by overheating, there may be no way of knowing of the failure of the sensing element before liquid is lost or before substantial damage to the vessel occurs from underfilling or overfilling. A self-protected liquid detector, one wherein the sense element is protected from overheating would have an obvious advantage over unprotected circuits, namely reliable operation despite operator negligence.
It is therefore an object of the present invention to detect the presence of a liquid in a vessel.
Another object is to detect the presence of very low temperature liquids.
Another objective is to provide over-temperature protection for a temperature sensitive resistance used to measure the temperature of a liquid in an evacuated container.